The present invention relates to a process for producing pipe having one or more layers of thermoplastic resin, and preferably having three or more layers comprising at least one outer layer, an intermediate layer, and at least one inner layer.
Many attempts have been made to produce a multiple layered pipe in order to combine the desirable characteristics of different thermoplastic resins. These attempts have usually involved extruding several streams of thermoplastic resins through a series of concentric tubes which are fixed relative to one another in a radial manner, such as by spiders, to define annular passages therebetween, and then subsequently joining the resultant annular layers to produce a multiple layered pipe. Pipes prepared by such processes and apparatus, however, have had spider marks thereon due to the flow interruptions caused by the many spiders necessitated in such apparatus. Moreover, such apparatus contain adjusting screws which protrude into the individual die passages for adjusting the thickness of the annular layers. These screws also interfere with the flow of the molten resin as it is being extruded causing additional marks on the pipe.
Furthermore, it is undesirable to provide an extrusion die for each layer of the desired pipe. Since high internal pressure is required for the extrusion of the highly viscous heat plastified material, such apparatus is subject to distortions in the die which cause undesirable nonuniformity in the thickness of each layer, unless the extrusion pressures of each layer are balanced. However, in practice, it is frequently difficult to maintain extrusion pressures constant in their proper relationship. Those types of apparatus employing a separate die for each layer thus inherently incur difficulty in controlling the thickness of each layer in the desired pipe.
Another disadvantage associated with conventional processes for the production of co-extruded plastic pipe is their inability to equalize the flow of thermoplastic within the die so that it is applied as a continuous layer of constant thickness. This problem is particularly prevalent when layers of thermoplastic are extruded through annular orifices to accomplish multiple layer laydown. In order to form a layer of constant thickness, it is absolutely essential that the flow of melt through the annular orifice be constant about its entire circumference.
In the extrusion of multiple layered pipe having an intermediate thin film layer of thermoplastic material, it is necessary at the point of laydown that the layers of thermoplastic material have viscosity indexes similar to that of the film layer. Any significant difference in viscosity will usually prevent bonding of the outer and inner layers to the film. In addition to controlling the relative viscosities, it is also necessary that each layer have a constant thickness. Failure to achieve very precise viscosity control and uniformity of thickness of each layer in pipe having one or more thin film layers, results in a product which has inferior physical properties.
U.S. Pat. No. 4,249,875 by Hart et al discloses a process and apparatus for producing multiple layered pipe utilizing a single annular die passage to lay down multiple inner and/or outer layers of thermoplastic polymer onto a main stream of thermoplastic polymer. An extrusion passage of suitable length is utilized to allow the bonded thermoplastic to reunite from the interruptions caused by the radial support legs of the mandrel positioned in the extrusion passage. This reuniting of the thermoplastic occurring at the same time as the laydown of the inner and outer layers may result in some lack of uniformity in layer thickness. In addition, the Hart patent discloses separate extruders to supply thermoplastic material to the inner and outer layers respectively. The use of separate extruders makes it difficult to control the relative viscosities of the inner and outer layers because of the difficulty of operating both extruders at the same temperature.
It would be desirable, therefore, if a process for the production of either single layer or multiple layered thermoplastic pipe was available which could attain very precise control of both the layer thickness and viscosity of the thermoplastic.